Electric field gradient sensor

ABSTRACT

An electric field gradient sensor is presented, having a sensor body having an outer surface; and a plurality of electrodes distributed over the outer surface, each electrode having an electrode surface facing outward from the surface. The plurality of electrodes are arranged forming a plurality of electrode pairs, each electrode pair formed by a first electrode and a second electrode located on opposite sides of the sensor body. This sensor enables three-dimensional measurements of the electric field gradient along structures located in an electrically conductive medium, such as subsea structures, for example for monitoring the cathodic protection of such structure.

FIELD OF THE INVENTION

The present invention relates to an electric field gradient sensor formeasuring the electric field around and/or along a structure located inan electrically conducting medium. The electric field gradient sensor isin particular useful for assessing the state of cathodic protection ofpipelines, marine and/or subsea structures.

BACKGROUND ART

Structures, such as pipelines, marine structures, etc., located underwater, e.g. in sea, are often provided with cathodic protection forpreventing corrosion of the structure. However, the cathodic protectionmay become damaged or degrade with time, or be otherwise defective.Therefore, there exists a need to assess and/or monitor the status ofthe cathodic protection.

Measurements of the status of cathodic protection have conventionallybeen performed by contact measurements using a stab or probe steppedover the structure. This is however a rather time consuming procedure.

Faster measurements can be achieved by sensors configured for performingnon-contact measurements while being moved along the structure.

WO 2017/126975 A1 presents a method and a sensor for detection ofelectric fields around a structure in an electrically conducting mediumwhile moving the sensor along the structure. This sensor comprises twoelectrodes located on a rotating disc, enabling measuring the electricalfield in the rotation plane of the disc. For measuring the electricfield in three dimensions two sensors are required, arranged at a 90°angle to one another. This leads to a complex instrument, relying onrotating parts and having complex electronics inside. The instrumentfurther has a relatively large weight, which has to be taken intoaccount when using the sensor.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electric field gradientsensor having a less complex construction. Also, it is an object of thepresent invention to provide a sensor that is more suitable for subseause wherein high pressures and being immersed in anelectrically-conductive liquid is a challenge.

In particular, it is an object of the invention to provide an electricfield gradient sensor enabling three-dimensional measurements of theelectric field gradient along a structure.

This is achieved by an electric field gradient sensor as defined inclaim 1.

Embodiments of the invention are claimed in dependent claims.

In a first aspect an electric field gradient sensor is provided,comprising:

-   -   a sensor body having an outer surface; and    -   a plurality of electrodes distributed over said surface, each        electrode having an electrode surface facing outward from said        surface;        wherein said plurality of electrodes are arranged forming a        plurality of electrode pairs, each electrode pair comprising a        first electrode and a second electrode located on opposite sides        of said sensor body.

By measuring the differential voltage between the first and the secondelectrode of each electrode pair, an electric field vector can becalculated. By the arrangement of the plurality of electrode pairs athree-dimensional measurement of the electric field can be obtained. Bysampling the differential voltages while moving the sensor along and/oraround at least a portion of a structure, a three-dimensional map of theelectric field gradient can be obtained, in a relatively fast manner.

This sensor is of a simple and light weight construction, and can bemanufactured at low cost, as will be apparent further below. As movingparts are avoided, a sensor is achieved having low complexity, whileenabling three-dimensional measurement of the electric field.

The electrodes are preferably arranged with their surfaces arrangedflush with the outer surface of the sensor body.

For each electrode pair an interconnecting line can be formed,interconnecting the first electrode and the second electrode. Theelectrodes are preferably distributed over said surface such that saidinterconnecting lines intersect at one single point of intersection.This point of intersection forms a reference point for the differentialvoltages measured across each electrode pair. By all electrode pairshaving a common reference point, the calculation of the electric fieldgradient from the measured differential voltages can be simplified.

The single point of intersection may substantially correspond to ageometrical center of said sensor body. This also contributes toavoiding unnecessarily complex calculations.

The shape and the size of the sensor body can be selected in accordancewith, e.g., the intended use of the sensor. For example, the size and/orthe shape can be selected based on the location and/or structure whichwill be assessed using the sensor, and/or the specifics of the vehicle,such as an ROV or AUV, with which the sensor will be used.

The sensor body may have a substantially spherical shape. This provideseasy manufacturing and enables, by the distribution of the electrodesover the outer surface, measuring the electric field in threedimensions. A spherical shape of the sensor body may have the advantageof less complex calculations of the electrical field from thedifferential voltages measured over each electrode pair.

Alternatively, the sensor body may have an oblate spheroid shape or aprolate spheroid shape.

Alternatively, the sensor body may have a substantially cylindricalshape. In this embodiment, the electrodes are distributed over thecurved cylinder surface. The electrodes, provided on the curved surface,are further preferably distributed over the length of the cylinder,having a plurality of electrodes along the length of the cylinder. Acylindrically shaped sensor body may be advantageous for use with smallAUVs (autonomous underwater vehicles), which may be configured for themounting of cylindrical payload modules thereto.

The sensor preferably further comprises sensor electronics arrangedwithin the sensor body for measuring a voltage over each of theelectrode pairs. Alternatively, the sensor electronics may be arrangedin an unmanned underwater vehicle to which the sensor is mounted duringuse, the sensor electronics being connected to the electrodes viaelectrical connectors extending from the electrodes, through theinterior of the sensor body and to the sensor electronics in the vehiclevia mounting means via which the sensor is mounted to the vehicle.

The sensor electronics is preferably pressure tolerant or pressureresistant. This can be achieved, for example, by potting theelectronics. Thereby, a pressure housing is not required, whereby theweight of the sensor can be reduced.

The voltage difference measured over an electrode pair may berepresented by a vector, indicating the value and/or magnitude of thedifferential voltage measured over the first and the second electrode,and having a direction defined by the line of intersection between thefirst and second electrode. The resulting electric field vector at ameasurement, or sampling, location is obtained by combining the vectorsachieved from each electrode pair. An electric field vector at themeasurement or sampling point can be calculated by combining the vectorsassociated by the individual electrode pairs by vector addition or by amore complex equation, depending e.g. on the geometry of the sensor bodyand the locations and/or the distribution of the electrodes. Byperforming measurements at a plurality of measurement, or sampling,points around or along at least a part of the structure, the electricfield gradient around or along the structure can be calculated. From theelectric field gradient, the status of the cathode protection of thestructure can be determined, whereby a defective or degraded cathodeprotection can be detected.

The sensor electronics preferably comprises electrical contacts to eachof the plurality of electrodes and a microcontroller for sampling adifferential voltage over each electrode pair.

The sensor electronics preferably further comprises one or more of thefollowing: amplifiers for amplifying measured voltages, a power source,and a communication unit for communicating said measured voltages to areceiver arranged remote from the sensor.

The sensor electronics is hence relatively simple, and allows fastinterrogation of the differential voltages over the plurality ofelectrode pairs.

The sensor preferably further comprises a bias electrode arranged forsetting a bias voltage for the sensor electronics. The bias electrodemay advantageously be arranged at the point of intersection of the linesinterconnecting the electrodes of the electrode pairs. Alternatively,the bias electrode may be arranged outside of said sensor body, forexample at a center location in front of or on the sensor body.

The sensor body preferably comprises a non-conductive plastic orcomposite material. This enables easy manufacturing and low cost, aswell as a light weight sensor. In preferred embodiments, as will bedescribed below, the sensor body may be manufactured by 3D printing, inparticular if the sensor body is of spherical or spheroidal shape. Thenon-conductive plastic material may comprise, for example, a reinforcedepoxy material. Other alternatives may include composite glass.

The electrode surfaces may comprise gold, carbon, platinum, titanium orstainless steel. These materials have been seen to function well whilebeing immersed in a conductive fluid, such as e.g. (sea) water. Thesematerials are further easy to apply during manufacturing, and at leastgold, carbon, platinum and titanium require little maintenance.

In preferred embodiments, the electrodes may be formed by gold platedcircuit boards. This offers easy and cost-effective manufacturing.Alternatively, the electrodes may be formed from metal plated with goldor platinum.

In some embodiments, each electrode may be provided with an electricallynon-conductive tube, a first end of said tube enclosing said electrode.The tubes extend outwards from the sensor body, preferably in asubstantially radial direction.

That is, the sensor may be provided with a plurality of hollownon-conductive tubes, or sleeves, extending radially outwards from thesensor body. The tubes are attached, or sealed, to the sensor body suchthat each electrode is located within a first end of the tube, i.e.,enclosed by a tube. By adding such non-conductive tubes to the sensor,the effective distance between the electrodes of each electrode pair isincreased, increasing the differential voltage measured over theelectrode pair. Thereby, the sensitivity of the sensor can be increased.

The non-conductive material may be, for example, a plastic material or aflexible polymer or rubber. This material is preferably chosen such thatthe tubes have a flexibility such as to be deformed without damage ifcolliding when an object. At the same time, the tubes should be rigidenough not to deform under normal use, i.e., when travelling or beingmoved through the electrically conductive medium, such as seawater,without colliding with an object during measurement.

The number of electrodes may be selected as a balance betweenmeasurement accuracy and the cost and complexity of manufacturing thesensor. The sensor typically comprises between 6 to 40 electrodes,preferably between 20 to 34, more preferably 24 or 32 electrodes. Thus,at least three electrode pairs should be provided in order to enablemeasuring the electric field in three dimensions. Providing 12 electrodepairs on the sensor body may provide measurement results of higheraccuracy, while still offering a fast measurement.

The sensor body may be hollow and provided with a plurality of holes inits outer surface. Thereby, the liquid or fluid, such as water, throughwhich the sensor is moved can be allowed to enter the sensor body. Ifthe bias electrode is arranged within the sensor body, the liquid orfluid can thereby come into contact with the bias electrode, for settinga bias voltage.

The sensor further preferably comprises a mounting component, such as amounting pole, coupled to said sensor body for mounting said sensor to avehicle, in particular to an unmanned underwater vehicle. Such unmannedunderwater vehicle will typically be an ROV (remotely operated vehicle)or an AUV (autonomous underwater vehicle), which are known in the art.The mounting pole is preferably made of a non-conductive compositematerial or a plastic material.

The mounting pole generally has a length sufficient to position thesensor at such a distance from the vehicle such that any disturbancesfrom the vehicle and any electronics and/or other equipment orcomponents arranged thereon or therein are minimized, at least such asto be negligible. Such distance typically amounts to between 0.5 to 1.5meters, preferably about 1 meter.

The sensor is preferably fixed with respect to the vehicle duringmeasurements. Thereby, moving parts are avoided, leading to a sensorhaving low complexity.

In a second aspect, a system for measuring an electric field gradient ata structure located in an electrically conductive medium is provided,the system comprising an electric field gradient sensor according to thefirst aspect mounted to said unmanned underwater vehicle.

In a third aspect, a method for manufacturing an electric field gradientsensor is provided, comprising the steps of:

-   -   providing sensor electronics;    -   forming a sensor body around said sensor electronics, said        sensor body having an outer surface; and    -   forming a plurality of electrodes distributed over said surface,        each electrode having an electrode surface facing outward from        said surface and being electrically connected to said sensor        electronics;        wherein said plurality of electrodes are arranged such as to        form a plurality of electrode pairs, each electrode pair        comprising a first electrode and a second electrode located on        opposite sides of said sensor body.

In particular, the sensor according to the first aspect may bemanufactured by the method according to the third aspect. The variousembodiments and associated technical effects and advantages describedabove with reference to the first aspect apply analogously and/orcorrespondingly to the method of the third aspect.

The sensor body may preferably be made, at least partially, from aplastic and/or composite material. This provides a light weight sensor.The sensor electronics may be potted inside the sensor body.

The sensor body is advantageously formed by 3D printing. That is, thesensor body may be 3D printed around the sensor electronics. This offersan easy and cost effective way of manufacturing the sensor. Furthermore,the sensor body can be formed in accordance with the intended use.

Alternatively, the sensor body may be formed by molding or machining,i.e., molding a plastic material around the sensor electronics.

The step of forming the plurality of electrodes preferably comprisesproviding electrodes having the outward facing electrode surfacecomprising gold, carbon, platinum, titanium or stainless steel.

The electrode surface may be formed by plating gold or platinum onto ametal.

The step of forming the plurality of electrodes may comprise platingcircuit boards with gold. Alternatively and/or additionally, theelectrodes may be formed by printing carbon onto gold plated circuitboards.

As described above, the sensor may be provided with non-conductive tubesenclosing each electrode and extending radially from the sensor body.

In a fourth aspect, a method is provided of performing electric fieldgradient measurements of a structure located in an electricallyconducting medium, comprising the steps of:

-   -   providing an electric field gradient sensor according to the        first aspect;    -   mounting said sensor to an unmanned underwater vehicle; and    -   moving said vehicle along at least a part of said structure,        while sampling differential voltages over said electrode pairs        of said sensor at a plurality of sampling locations.

This method enables a time- and cost-effective measurement of theelectric field gradient in the vicinity of the structure, enabling athree-dimensional measurement of the electric field gradient whileflying over and/or moving the sensor along at least a part of thestructure. This enables a relatively quick manner of e.g. assessing thecathodic protection of the structure.

Structures include for example flowlines, pipelines and jackets arrangedin an electrically conducting medium, for example water, e.g. sea water,and other subsea structures. Other locations and/or other types ofelectrically conductive media are also possible.

The method, and the sensor, enables measurement of anode current of acathodic protection installation, current densities, current drain toother structures, as well as well as detection of defects in a coatingon the structure.

The electric field gradient sensor may be a sensor according to any oneor more of the embodiments of the sensor of the first aspect describedabove. As described above, the vehicle may be any unmanned vehicle suchas commercially available ROV or AUV.

The sensor is preferably maintained substantially fixed with respect tosaid vehicle. Since no moving parts are involved in the sensor, a lesscomplex system is achieved.

The differential voltages can be combined to form an electric fieldgradient vector at each of said plurality of sampling locations.

The method preferably further comprises registering the position of eachsampling location. Thereby, the measurement results can be correlatedwith the location at which they were obtained, e.g. such as to correlatethem with the geometry of the structure.

A processor including a computer program for processing the measurementresults in order to calculate the electric field gradient at thesampling locations, and storing it, preferably together with thesampling location, may be provided on the vehicle or at a remotelocation.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the description of the invention by way of non-limiting andnon-exclusive embodiments. These embodiments are not to be construed aslimiting the scope of protection. The person skilled in the art willrealize that other alternatives and equivalent embodiments of theinvention can be conceived and reduced to practice without departingfrom the scope of the present invention. It can further be noted thatthe drawings are not necessarily drawn to scale.

Embodiments of the invention will be described with reference to thefigures of the accompanying drawings, in which like or same referencesymbols denote like, same or corresponding parts, and in which:

FIG. 1 a shows a perspective view of an electric field gradient sensoraccording to an embodiment;

FIG. 1 b schematically illustrates the relative position of theelectrodes of a sensor as shown in FIG. 1 a according to an embodiment;

FIGS. 1 c and 1 d schematically illustrate a sensor according to afurther embodiment being a further development of the sensor shown inFIGS. 1 a and 1 b;

FIG. 2 a shows a perspective view of an electric field gradient sensoraccording to another embodiment;

FIG. 2 b schematically illustrates the relative position of theelectrodes of a sensor as shown in FIG. 2 a according to an embodiment;

FIG. 2 c schematically illustrates a sensor as shown in FIG. 2 a mountedto an unmanned underwater vehicle according to an embodiment;

FIG. 3 schematically illustrates the electric field gradient sensor inuse for measuring cathodic protection of a structure according to anembodiment; and

FIG. 4 schematically illustrates an electronic scheme of the sensoraccording to an embodiment.

DESCRIPTION OF EMBODIMENTS

FIG. 1 a shows a non-limiting embodiment of an electric field gradientsensor according to an embodiment of the invention. The sensor 1comprises a sensor body 2 provided with a plurality of electrodes 4 onits outer surface. The electrodes 4 are arranged to form electrodepairs, each electrode pair being formed by oppositely locatedelectrodes. The surfaces of the electrodes 4 contact the conductivemedium, e.g. water, in which the sensor is immersed when performingmeasurements.

By measuring the voltage difference over each electrode pair, anelectric field vector can be calculated, as has been described in detailin the Summary of invention herein above. By the distribution of theelectrode pairs over the surface of the sensor body, the electric fieldgradient in the vicinity of a structure can be measured in threedimensions while moving the sensor along and/or around the structure.

The first electrode and the second electrode of the electrode pair arelocated on opposite sides of the sensor body 2, such that the electrodesurfaces face in substantially opposite directions.

In the embodiment shown in FIG. 1 a, the electrodes 4 are advantageouslyarranged such that lines interconnecting oppositely located electrodesintersect at a single point within the sensor body 2. This concept isdescribed herein below with reference to FIG. 1 b.

Further, in the embodiment of FIG. 1 a, sensor electronics 10 areprovided inside the sensor body 2. An example of such sensor electronics10 is illustrated in FIG. 4 . Alternatively, the sensor electronics 10may be provided in the unmanned underwater vehicle, and connected to theelectrodes 4 via electrical connectors.

The sensor 1 is further provided with a mounting pole 12, coupled to thesensor body 2 for mounting the sensor to an unmanned underwater vehicle,as shown in FIG. 3 .

The electrodes 4 preferably arranged with their surfaces flush with theouter surface of the sensor body 2, their surfaces in contact with themedium, e.g. water, in which the measurements are performed.

The sensor body 2 may be hollow, and provided with holes 14. Thereby,the fluid, generally water, in which the sensor is immersed may comeinto direct contact with the bias electrode 8 located within the sensorbody 2.

FIG. 1 b provides a simplified schematic illustration of three electrodepairs 4 a, 4 b, 4 c. Each electrode pair comprises two electrodes,located on opposite sides of the sensor body 2 and interconnected byimaginary interconnection lines i1, i2, i3. In the embodiment of FIG. 1b, these interconnection lines i1, i2, i3 have a common, singleintersection point 6. This point 6 forms a common reference point forthe electrode pairs. As also illustrated in the embodiment of FIG. 1 b,the intersection point 6 is advantageously located at the center of thespherical body 2.

It should be noted that the illustration of FIG. 1 b provides asimplified illustration, for illustrating the interconnecting lines ofthe electrodes of each electrode pair having a common point ofintersection. While in FIG. 1 b the electrodes appear located in onesingle plane, in preferred embodiments, the electrodes will bedistributed over the surface of the sensor body, as illustrated in FIG.1 a. Furthermore, in FIG. 1 b three electrode pairs are illustrated,which may be sufficient in some embodiments. However, preferably moreelectrode pairs are provided, as also illustrated in FIG. 1 a. Asdescribed above, providing 12 electrode pairs on the sensor provides agood balance between manufacturing cost and measurement accuracy.

The sensor preferably further comprises a bias electrode 8, which in theembodiment illustrated in FIG. 1 b is arranged at the point ofintersection 6. The bias electrode 8 is electrically connected to thesensor electronics 10, arranged within the sensor body 2.

FIGS. 1 c and 1 d schematically shows a sensor 1′ according to a furtherdevelopment of the sensor 1 shown in FIGS. 1 a and 1 b. According tothis embodiment, the sensor is provided with a plurality of electricallynon-conductive tubes 34, attached, e.g. sealed, around each electrode 4and extending substantially radially outwards from the sensor body 2.The concept is illustrated in FIG. 1 c, for one electrode pair. Itshould be understood that a tube 34 is provided at each electrode 4, asshown in FIG. 1 d.

By providing the tubes 34, the effective distance between the electrodes4, influencing the differential voltage, or potential, measured acrossthe electrodes 4 forming an electrode pair, is increased. During use, asthe sensor is moved through a medium, typically seawater, in thepresence of an electric field, the differential voltage between theelectrodes of each electrode pair is measured. From this differentialvoltage, as described further herein below, a measure of the electricfield gradient in the vicinity of a structure, whose cathodic protectionis to be monitored or assessed, is determined. In the absence of thetubes, a voltage V1 is measured, having a value correspond to theelectric field multiplied by the distance between the electrodes 4,which corresponds to the diameter of the sensor body 2. However, in thepresence of the non-conductive tubes 34, the differential voltage V2 ismeasured, corresponding to the voltage difference between the locationsof the outer ends of the tubes 34. Thereby, the size of the sensor body2 is artificially extended, increasing the value of the differentialvoltage measured and hence the sensitivity of the sensor 1. Due to theflexibility of the tubes 34, these will deform if the sensor bodycollides with an object during its movement around the structure to beassessed. Thereby, the passage of the sensor through a narrower passagewill not be obstructed.

The concept illustrated in FIGS. 1 c and 1 d can be applied analogouslyto the embodiment of FIGS. 2 a and 2 b described herein below.

FIGS. 2 a and 2 b show an electric field gradient sensor 201 accordingto a second embodiment of the invention. The sensor 201 comprises asensor body 202 having a cylindrical shape, where the plurality ofelectrodes 204 are arranged on the curved surface 203 of the cylindricalsensor body 202, and distributed over the length of the cylinder. Exceptfor this, the features of the sensor 201 and the function thereofcorrespond to those described with reference to FIG. 1 a.

As schematically illustrated in FIG. 2 b , the electrodes 204 mayadvantageously be arranged such that interconnection lines i201, i202 ofelectrodes making up electrode pairs 204 a, 20 ab intersect at anintersection point 206. Although in FIG. 2 b , for ease of illustration,only two such interconnection lines i201, i202 are shown, it should beunderstood that this would apply for all electrode pairs of the sensor201.

As mentioned above, a cylindrically shaped sensor has the advantage thatit may be mounted to an ROV or AUV as a cylindrical payload section.FIG. 2 c illustrates an unmanned underwater vehicle 216 having acylindrical electric field gradient sensor 201 mounted thereto in thepayload region 217.

FIG. 3 shows the electric field gradient sensor 1 mounted to an unmannedunderwater vehicle 16 for assessing the state of a cathodic protection,here provided in the form of anodes 18, of a pipeline 20. As the sensor1 is moved along the pipeline 20, or at least a part thereof, in adirection 22, the differential voltages over the electrode pairs, formedby the electrodes 4, are sampled at regular and/or preset intervals orlocations.

The vehicle 16 will typically be an ROV (remotely operated vehicle) oran AUV (autonomous underwater vehicle), which are known in the field.

As illustrated in FIG. 3 , the sensor 1 may be mounted in front of thevehicle 16 via the mounting pole 12. The sensor is mounted at such adistance to the vehicle that the vehicle, and instruments providedthereon or therein, do not disturb the measurements performed by thesensor. Such distance may for example be about 1 meter.

The sensor 1 is fixed with respect to the vehicle 16 duringmeasurements. That is, no parts of the sensor are rotating or otherwisemoving with respect to the vehicle as the vehicle is moved along thepipeline 20 while performing measurements of the electric field gradientalong the pipeline 20.

Although in FIG. 3 the structure whose cathodic protection is to beassessed or monitored is illustrated as a pipeline, the method can beapplied analogously to any other structure, such as a marine or a subseastructure.

By registering the measured differential voltages, and/or the calculatedresulting electric field gradient, obtained at each sampling point,together with the location of the sampling point, a three-dimensionalrepresentation of the electric field gradient along and/or around thepipeline 20 can be obtained. The registration of the measurement resultsassociated with the sampling locations can be performed by a processingunit 23 mounted to the vehicle 16. Alternatively, the processing unitmay be located at a location remote from the vehicle 16 and sensor 1,e.g. at an on-shore location from which the vehicle 16 and sensor 1 aredeployed.

FIG. 4 schematically shows an embodiment of the electronics scheme ofthe electric field gradient sensor. This can be applied to the sensoraccording to the embodiments of FIGS. 1 a, 1 b and of FIGS. 2 a , 2 b.

According to the embodiment illustrated in FIG. 4 , the sensorelectronics 10, which is arranged within the sensor body 2, 202,comprises a microprocessor 24 and amplifiers 26, which are electricallyconnected to the electrodes 4 (or 204) and the bias electrode 8 viaelectrical wiring. Thereby, the microprocessor can perform sampling ofthe differential voltage over each electrode pair, at regular and/orpre-set intervals, as described above. The microprocessor preferablycomprises a clock unit for time-stamping the sampling results. Thesensor electronics 10 further comprises a communication unit 28 fortransmitting the sampling results to a receiver located remote from thesensor, for example on the ROV or AUV to which the sensor is mounted, orto more remote location, such as an operator control location. Asdescribed above, the sampling results may be combined with informationdefining the sampling locations, for example in the form of signals froma navigation unit, such as an inertial navigation system, INS, or anultra-short baseline, USBL, system, located on board the ROV or AUV.Alternatively, the sampling locations can be determined via the timestamps associated with the sampling results. The sensor electronicsfurther comprises a power unit 30, such as a battery, for providingpower to the components of the sensor electronics.

It should be understood that further components may be added to thesensor electronics, as will be understood by the person skilled in theart.

The bias electrode 8, electrically coupled to the sensor electronics 10,can be located within the sensor body 2, 202, for example at the centerof it, as described above. Alternatively, the bias electrode 8 may belocated on the outer surface of the sensor body, for example at thefront center point thereof.

It will be clear to a person skilled in the art that the scope of theinvention is not limited to the examples discussed in the foregoing, butthat several amendments and modifications thereof are possible withoutdeviating from the scope of the invention as defined in the attachedclaims. While the invention has been illustrated and described in detailin the figures and the description, such illustration and descriptionare to be considered illustrative or exemplary only, and notrestrictive. The present invention is not limited to the disclosedembodiments but comprises any combination of the disclosed embodimentsthat can come to an advantage.

Variations to the disclosed embodiments can be understood and effectedby a person skilled in the art in practicing the claimed invention, froma study of the figures, the description and the attached claims. In thedescription and claims, the word “comprising” does not exclude otherelements, and the indefinite article “a” or “an” does not exclude aplurality. In fact it is to be construed as meaning “at least one”. Themere fact that certain features are recited in mutually differentdependent claims does not indicate that a combination of these featurescannot be used to advantage. Any reference signs in the claims shouldnot be construed as limiting the scope of the invention. Features of theabove described embodiments and aspects can be combined unless theircombining results in evident technical conflicts.

1. An electric field gradient sensor, comprising: a sensor body havingan outer surface; and a plurality of electrodes distributed over saidsurface, each electrode having an electrode surface facing outward fromsaid surface; wherein said plurality of electrodes are arranged forminga plurality of electrode pairs, each electrode pair comprising a firstelectrode and a second electrode located on opposite sides of saidsensor body.
 2. The sensor according to claim 1, wherein each electrodeis provided with a non-conductive tube, a first end of said tubeenclosing said electrode.
 3. The sensor according to claim 2, whereinsaid non-conductive tubes extend in a substantially radial directionoutwards from said sensor body.
 4. The sensor according to claim 2,wherein said non-conductive material comprises a plastic material, aflexible polymer or rubber.
 5. The sensor according to claim 1, whereinfor each electrode pair an interconnecting line is formedinterconnecting the first electrode and the second electrode, andwherein said electrodes are distributed over said surface such that saidinterconnecting lines intersect at one single point of intersection. 6.The sensor according to claim 5, wherein said single point ofintersection substantially corresponds to a geometrical center of saidsensor body.
 7. The sensor according to claim 1, wherein said sensorbody has a substantially spherical shape.
 8. The sensor according toclaim 1, wherein said sensor body has an oblate spheroid shape or aprolate spheroid shape.
 9. The sensor according to claim 1, wherein saidsensor body has a substantially cylindrical shape.
 10. The sensoraccording to claim 1, further comprising sensor electronics formeasuring a voltage over each of said electrode pairs, said sensorelectronics comprising electrical contacts to each of said plurality ofelectrodes.
 11. The sensor according to claim 10, wherein said sensorelectronics is arranged within said sensor body or at a vehicle to whichthe sensor is mounted.
 12. The sensor according to claim 10, whereinsaid sensor electronics further comprises a microcontroller for samplinga differential voltage over each electrode pair.
 13. The sensoraccording to claim 12, wherein said sensor electronics further comprisesone or more of the following: amplifiers for amplifying measuredvoltages, a power source, and a communication unit for communicatingsaid measured voltages to a receiver arranged remote from said sensor.14. The sensor according to claim 10, further comprising a biaselectrode arranged for setting a bias voltage for said sensorelectronics.
 15. The sensor according to claim 14, wherein eachelectrode is provided with a non-conductive tube, a first end of saidtube enclosing said electrode, wherein said bias electrode is arrangedat said single point of intersection.
 16. The sensor according to claim13, wherein said bias electrode is arranged outside of said sensor body,preferably at a center location in front of the sensor body.
 17. Thesensor according to claim 1, wherein said sensor body comprises aplastic material and/or a composite material.
 18. The sensor accordingto claim 1, wherein said electrode surfaces comprise gold, carbon,platinum, titanium or stainless steel.
 19. The sensor according to claim17, wherein said electrodes are formed by gold plated circuit boards,gold plated circuit boards printed with carbon; or by metal plated withgold or platinum.
 20. The sensor according to claim 1, wherein saidplurality of electrodes comprises between 6 to 40 electrodes.
 21. Thesensor according to claim 1, further comprising a mounting componentcoupled to said sensor body for mounting said sensor to a vehicle, inparticular to an unmanned underwater vehicle.
 22. The sensor accordingto claim 1, wherein said sensor body is hollow and is provided with aplurality of holes in its outer surface.
 23. A system for measuring anelectric field gradient at a structure located in an electricallyconductive medium, the system comprising: an electric field gradientsensor comprising a sensor body having an outer surface and a pluralityof electrodes distributed over said surface, each electrode having anelectrode surface facing outward from said surface; an unmannedunderwater vehicle, wherein the electrical field gradient sensor insmounted to the unmanned underwater vehicle; and sensor electronics formeasuring a voltage over each of said electrode pairs, said sensorelectronics comprising electrical contacts to each of said plurality ofelectrodes; wherein said plurality of electrodes are arranged forming aplurality of electrode pairs, each electrode pair comprising a firstelectrode and a second electrode located on opposite sides of saidsensor body.
 24. The system according to claim 23, wherein said sensorelectronics is arranged within said sensor body or in said vehicle. 25.The system according to claim 23, wherein for each electrode pair aninterconnecting line is formed interconnecting the first electrode andthe second electrode, and wherein said electrodes are distributed oversaid surface such that said interconnecting lines intersect at onesingle point of intersection.
 26. The system according to claim 25,wherein said single point of intersection substantially corresponds to ageometrical center of said sensor body.
 27. The system according toclaim 22, wherein said sensor electronics further comprises amicrocontroller for sampling a differential voltage over each electrodepair.
 28. The system according to claim 27, wherein said sensorelectronics further comprises one or more of the following: amplifiersfor amplifying measured voltages, a power source, and a communicationunit for communicating said measured voltages to a receiver arrangedremote from said sensor.
 29. The sensor according to claim 23, furthercomprising a bias electrode arranged for setting a bias voltage for saidsensor electronics.
 30. The system according to claim 29, wherein foreach electrode pair an interconnecting line is formed interconnectingthe first electrode and the second electrode, and wherein saidelectrodes are distributed over said surface such that saidinterconnecting lines intersect at one single point of intersection,wherein said bias electrode is arranged at said single point ofintersection.
 31. The system according to claim 29, wherein said biaselectrode is arranged outside of said sensor body, preferably at acenter location in front of the sensor body.
 32. The system according toclaim 23, wherein said sensor body comprises a plastic material and/or acomposite material.
 33. The system according to claim 23, wherein saidsensor body is hollow and is provided with a plurality of holes in itsouter surface.
 34. A method for manufacturing an electric field gradientsensor, comprising the steps of: providing sensor electronics; forming asensor body around said sensor electronics, said sensor body having anouter surface; and forming a plurality of electrodes distributed oversaid surface, each electrode having an electrode surface facing outwardfrom said surface and being electrically connected to said sensorelectronics; wherein said plurality of electrodes are arranged such asto form a plurality of electrode pairs, each electrode pair comprising afirst electrode and a second electrode located on opposite sides of saidsensor body.
 35. The method according to claim 34, wherein for eachelectrode pair an interconnecting line is formed interconnecting thefirst electrode and the second electrode, and wherein said electrodesare distributed over said surface such that said interconnecting linesintersect at one single point of intersection.
 36. The method accordingto claim 35, wherein said single point of intersection substantiallycorresponds to a geometrical center of said sensor body.
 37. The methodaccording to claim 34, wherein said sensor body is formed by 3Dprinting.
 38. The method according to claim 24, wherein said sensor bodyis formed by molding or machining.
 39. The method according to claim 34,wherein said sensor body comprises a plastic material or a compositematerial.
 40. The method according to claim 34, wherein said step offorming said plurality of electrodes comprises providing electrodeshaving the outward facing electrode surface comprising gold, carbon,platinum, titanium or stainless steel.
 41. The method according to claim40, wherein said electrode surface is formed by plating gold or platinumonto a metal.
 42. The method according to claim 34, wherein said step offorming said plurality of electrodes comprises plating circuit boardswith gold.
 43. The method according to claim 34, wherein said step offorming said plurality of electrodes comprises printing carbon onto goldplated circuit boards.
 44. The method according to claim 34, furthercomprising attaching a non-conducting tube to each of said electrodessuch that a first end of said tube encloses said electrode, said tubeextending radially outwards from said sensor body.
 45. A method ofperforming electric field gradient measurements of a structure locatedin an electrically conducting medium, comprising the steps of: providingan electric field gradient sensor according to claim 1; mounting saidsensor to an unmanned underwater vehicle; and moving said vehicle alongat least a part of said structure, while sampling differential voltagesover said electrode pairs of said sensor at a plurality of samplinglocations.
 46. The method according to claim 45, wherein said sensor ismaintained substantially fixed with respect to said vehicle.
 47. Themethod according to claim 45, wherein said differential voltages arecombined to form an electric field gradient vector at each of saidplurality of sampling locations.
 48. The method according to claim 45,further comprising registering the position of each sampling location.